The present invention relates to a method for treating a metal surface, which is offered for coating, with zinc phosphate. In detail, it relates to a treating method to make a zinc phosphate coating film which is suitable for an electrodeposition coating, especially, for a cationic electrodeposition coating, and which is superior in coating film adhesion, corrosion resistance, especially, warm brine resistance, and a property to prevent rust of a scab type (scab corrosion) (hereinafter, referred to as "scab resistance").
Metal materials have been used in various fields such as automobile bodies and other attachments, building materials, furniture etc. Metal is treated with zinc phosphate as coating pretreatment to prevent corrosion resulting from oxygen or sulfur oxides in the air, rainwater, seawater and so forth. A zinc phosphate film thus-formed is required to be superior in adhesion to a metal surface substrate, and also, to be superior in adhesion (secondary adhesion) to a coating film being made on the zinc phosphate film and also, it is required to have sufficient rust-preventability under a corrosive environment. In particular, since an automobile body is repeatedly exposed to contact with salt water as well as variation of dry and wet atmospheric conditions at a scar of the external plate, the scab resistance and a high order of warm brine resistance have been desired.
Recently, there has been increased a case of treating with zinc phosphate for a metallic material having two kinds or more of metal surfaces. For example, to elevate the corrosion resistance of after-coating in a case of the automobile body, a material plated by zinc or by a zinc alloy on only one face of a steel material is used. If a conventional zinc phosphate treatment is carried out on such a metal surface having both of an iron-based surface and a zinc-based surface simultaneously, the zinc-based surface is inferior in corrosion resistance and secondary adhesion when compared with the iron-based surface. Because of this, for example, in Japanese Official Patent Provisional Publication, showa 57-152472 etc., there has been proposed a method for making a zinc phosphate coating film suitable for an electrodeposition coating on the metal surface having both of an iron-based surface and a zinc-based surface simultaneously. In this method, at least one of a manganese ion having a concentration of from 0.6 to 3 g/l and a nickel ion having a concentration of from 0.1 to 4 g/l is contained in a treating bath in which concentrations of a zinc ion, a phosphate ion and a coating film-converting accelerator are controlled. Further, in Japanese Examined Patent Publication, showa 61-36588, there has been proposed an art which comprises adding a fluorine ion of 0.05 g/l or more together with a manganese ion in order to lower a treating temperature.
A material made by combining an aluminum material with an iron or a zinc material has been practically used in various fields such as automobiles, building materials, etc.
If a material of this kind is treated with a conventional acidic zinc phosphate treating solution used for a conventional iron or zinc material, the aluminum ion eluted to the treating solution accumulates and, when the amount of aluminum ions becomes certainly high, there is a problem that inferior chemical conversion occurs in an order of an aluminum material and iron material. Since the aluminum material has better corrosion resistance when compared with an iron or a zinc material, the amount of a zinc phosphate coating film forming on the aluminum material has not so far been prescribed. However, recently, because of an increasing demand for rust prevention, in order to enhance corrosion resistance of the aluminum material, a need to secure a coating film in a certain amount on the material surface has occurred.
Accordingly, to prevent an increase of aluminum ions in a treating solution, there has been proposed in Japanese Official Patent Provisional Publication, showa 57-70281, a method in which the aluminum ions are precipitated as K.sub.2 NaAlF.sub.6 or Na.sub.3 AlF.sub.6 by adding acid potassium fluoride and acid sodium fluoride into a treating solution. Also, there has been proposed in Japanese Official Patent Provisional Publication, showa 61-104089, a method in which the concentration of aluminum ions in a fluorine-based zinc phosphate treating solution is maintained at 70 ppm or less by controlling an area ratio of an aluminum-based surface to an iron-based surface at 3/7 or less.
The zinc phosphate treating method described in the Japanese Official Patent Provisional Publication, showa 61-104089, has a disadvantage so that an object to be treated is very limited, and also, it is difficult to maintain the concentration of aluminum ions at 70 ppm or less by only controlling the forementioned area ratio. On the other hand, the method described in the Japanese Official Patent Provisional Publication, showa 57-70281, does not limit an object to be treated, and it is superior in a point of adopting an idea such as aluminum ions in a treating solution is eliminated by precipitating them. However, a precipitate here formed shows a tendency of floating and suspending and attaches to a zinc phosphate coating film and makes the film ununiform. Because of this, in a case where electrodeposition coating is carried out on a zinc phosphate coating film, electrodeposition coating inferiority occurs and this inferiority becomes an origin of uniformity lack on a coating film and secondary adhesion inferiority of a coating film. Therefore, it is necessary to remove the precipitate of a floating and suspending character, but this removal is a complicate operation.